1. Field of the Invention
The present invention relates to image forming apparatuses of the electro-photographic type.
2. Description of the Related Art
For example, in the color image forming apparatus of the electro-photographic method used in copying machines, printers, etc., normally, primary toner images of different colors are formed on image retainers, the toner images so formed are transferred on to an intermediate image transfer member by primary image transfer thereby forming a color toner image, and subsequently a secondary image transfer is made of this color toner image on to a recording material. Image forming methods of this type are used very often in color image forming apparatuses that form color images.
The magnitude of the potential of the toner layer of a toner image on the intermediate image transfer member is determined by the amount of toner adhered. While the secondary transfer conditions are normally set matching with the magnitude of the potential of the toner image in a mostly dark image, if the amount of toner adhered is large, various transfer defects occur during the secondary image transfer process. As a result, image defects occur in the obtained image such as density fluctuations, toners splashing, etc. This is because the appropriate secondary transfer conditions differ depending on the magnitude of the toner layer potential. In particular, in color image forming, since the amount of toner adhered on the intermediate image transfer member becomes large, the problems of density fluctuations, toners splashing, etc., become pronounced.
As examples to solve the above problem, methods have been proposed in which a processing is done before secondary image transfer on the color toner image using a scorotron charger having a discharging electrode and a grid electrode by applying an electric charge with the same polarity as that of the toner potential to the color toner image process and thereby adjusting the toner layer potential of that color toner image to become uniformly high, and carrying out secondary image transfer in that condition.
In Japanese Patent Application Laid Open No. Hei 10-274892 and in Japanese Patent Application Laid Open No. Hei 11-143255, by making always constant the magnitude of the grid voltage applied to the grid electrode, the toner layer potential of the toner image to be subjected to secondary image transfer has been made uniformly high.
Further, in Japanese Patent Application Laid Open No. Hei 11-143255, it has been proposed to keep constant the difference between the potential of the toner image to be subjected to secondary image transfer and the potential of the secondary image transferring unit. In addition, in this disclosure, a measure has been taken to change the output of the pre-transfer charging unit according to the number of superimposed toner layers.
However, according to the experiments conducted on transfer conditions by the present inventors, the toner layer potential does not become constant when the charging is done with the constant grid voltage. Further, the toner layer potential varies not only with the quantity of toner forming the toner image on the intermediate image transfer member but also on the extent of toner charging, that is, the amount of electrical charge per unit mass of toner. Therefore, it became clear that it is difficult to prevent sufficiently the image defects occurring during secondary image transfer using the method of keeping constant the output of the pre-transfer charging unit or using the method of controlling the output of the pre-transfer charging unit according to the number of superimposed toner layers.
In particular, when carrying out pre-transfer discharging by lowering the magnitude of the toner layer potential during the processing before transfer, in the conventional control methods of keeping the grid voltage constant it became clear that it is difficult to prevent sufficiently the lowering of image quality caused by secondary image transfer.
In view of this, technology is being desired that solves the above problems in conventional technology and prevents the lowering of image quality caused by secondary image transfer.